I. Field of the Invention
This invention relates to a process of producing needle-shaped calcium carbonate particles which are useful as a filler or a reinforcing material of various materials such as rubbers, papers, plastics and paints.
II. Description of the Prior Art
The calcium carbonate actually used in industrial fields may be roughly divided into two gropus, i.e., heavy calcium carbonate and precipitated calcium carbonate. Heavy calcium carbonate is prepared by mechanically pulverizing limestone into fine particles. The shape of the heavy calcium carbonate particles is irregular and the particle size is in the range of several micrometers to 150 .mu.m.
On the other hand, precipitated calcium carbonate may be divided into two groups, i.e., so called light calcium carbonate with a particle size of 1-3 .mu.m and so called colloidal calcium carbonate with a particle size of 0.02-0.1 .mu.m. Precipitated calcium carbonate particles are currently manufactured by "liquid-liquid" process or by "liquid-gas" process. In the liquid-liquid process, a solution containing carbonate ion such as sodium carbonate and ammonium carbonate solution is reacted with a solution of a calcium compound such as calcium chloride and calcium acetate to produce calcium carbonate. In the liquid-gas process, a slurry of calcium hydroxide is reacted with carbon dioxide gas to produce calcium carbonate. In industrial fields, the liquid-gas process is usually employed and by changing the reaction conditions such as the concentration of the calcium hydroxide slurry, reaction temperature, manner of reaction and whether or not adding an additive, calcium carbonate particles with various shapes and sizes may be obtained. That is, calcium carbonate particles with a shape of spindle, pillar and cubic may be obtained. Further, a process of producing calcium carbonate particles having projections is disclosed in Japanese Patent Disclosure (Kokai) No. 30815/82 and a process of producing calcium carbonate particles having combined needle-shaped calcium carbonate particles is disclosed in Japanese Patent Disclosure (Kokai) No. 31530/82.
As to a process of producing needle-shaped calcium carbonate particles, a process employing the above-mentioned "liquid-liquid" process is disclosed in Japanese Patent Disclosure (Kokai) No. 203728/84, by which needle-shaped calcium carbonate particles with a length of 30-60 .mu.m and a diameter of 2-3 .mu.m are obtained. However, this process has not been industrially practiced.